1. Field of the Invention
The present invention relates to a method of and an apparatus for optically inspecting a semiconductor wafer for particular contamination.
2. Description of the Related Art
Processes for fabricating high-density integrated circuits (ICs) on Si wafers and GaAs wafers are-becoming more and more sophisticated year after year, and those integrated circuits are also becoming smaller and smaller in size. For these reasons, the contamination of semiconductor wafers with fine particles poses a serious problem. Because interconnections on semiconductor wafers have a width less than 5 .mu.m, the performance of IC chips is lowered and the yield of IC chips is reduced by contamination caused by fine particles whose size ranges from 0.5 .mu.m to several .mu.m, and particularly contamination by fine metal particles. It is therefore necessary to inspect wafers for particular contamination at various stages of semiconductor wafer fabrication. It is of importance to quickly remove wafers from the fabrication process and inspect the removed wafers, and also to inspect wafers according to nondestructive testing.
For example, for detecting a single fine particle having a size of 0.5 .mu.m on an 8-inch wafer, it is necessary to observe 10.sup.6 spots on the wafer in its entirety because of the size of a detectable observation field that is available. Due to the many observation spots, it is highly time-consuming to inspect the wafer, and inspection processes that can meet the requirements for quick observation are limited.
Prerequisites for analyzing wafers for fine particles thereon include freedom from contamination of the wafers and also freedom from damage to the wafers. It is also important for such a wafer analysis to determine the number, sizes, and types of fine particles that are present on the wafers.
One inspection process that is under consideration at present is to detect Mie scattering by fine particles for thereby detecting the fine particles. It will take more than 10 minutes to scan a wafer in its entirety according to the Mie scattering inspection process.
When a fine particle is detected, it is customary to identify the type of the detected fine particle. General elemental analytic processes for identify the type of a fine particle include an EPMA (Electron Probe Micro Analysis) process and an AES (Auger Electron Spectroscopy) process. Either of these elemental analytic processes requires that the wafer under inspection be placed in vacuum container and each particle be analyzed for its type for a period of about 5 minutes.
According to those elemental analytic processes, it is necessary to irradiate the wafer with an electron beam having an energy level of 10 keV or higher for identifying the type of a fine particle on the wafer, resulting in damage to the wafer. When irradiated with an electron beam, an organic fine particle on the wafer is decomposed into a carbon impurity which will remain on the wafer. Inasmuch as the fabrication of IC chips is carried out in an ultra-high clean room, the particle inspection apparatus itself is liable to cause contamination.
Due to the various above technical limitations, no practical particle inspection apparatus that can be used in the process of fabricating IC chips has been available in the art.
As described above, it takes more than 10 minutes to scan an 8-inch wafer in its entirety according to the Mie scattering inspection process, and it takes more than 5 minutes additionally to identify the type of a detected fine particle. The Mie scattering inspection process and the process for identifying the type of a detected fine particle cannot meet desired requirements for quick observation.
The EPMA and AES processes for identifying the type of fine particles are disadvantageous in that wafers under inspection are damaged by an electron beam for inspection, and the particle inspection apparatus itself tends to contaminate the clean room.